Fig. 2
(a) Ground velocity record of the M 3.9 aftershock and (b) electric field record, corresponding to the ground velocity. E
x
and E
y
denote the northward and the eastward components of electric field, respectively. (c) Absolute values of transfer function Z
ij
(i, j = 1, 2) derived from the observed ground velocity as the input and the observed electric field as the output. Dotted lines for Z11 and Z21 are standard errors. Dashed lines denote theoretical estimates for the resonance frequency of 9 Hz, corresponding to 
.